Generally, the upper die of a press is fitted to the bottom of a slide, while the lower die is fitted to the top of a bolster attached to a stationary machine frame. A feeder is connected with vertical reciprocation of a slide and feeds a workpiece between dies. Presses for causing reciprocation of the slide in the vertical direction using a linear motor have been increasingly made use of in recent years. Japanese Patent Laid-open No. 10-202397 discloses a press having six linear motors provided between an inner vertical surface of the machine frame and a vertical surface of the slide. This publication also discloses a press having two movers attached to the opposite vertical surfaces of connecting members. The connecting members are fixed to four guide posts extending vertically, and a slide is fixed to an upper part of the guide posts. Japanese Patent Laid-open No. 11-254191 discloses a press having four linear motors provided between an inner vertical surface of a machine frame and a vertical surface of a movable frame. Four guide posts extending vertically are fixed to the movable frame, and a slide is fixed to upper ends of the four guide posts. Japanese Patent Laid-open No. 2000-312992 discloses a press having a slide fixed to upper ends of the four vertically extending guide posts, and a pair of linear motors attached to each guide post.
Use of linear motors having excellent controllability brings a number of advantages to a press.    1. It is possible to obtain high precision because only a few components are required for power transmission.    2. Controlling a linear motor to allow high precision synchronization with peripheral devices such as a feeder for feeding a workpiece between dies is simple.    3. A pressure-stroke curve can be freely set using a numerical control unit.    4. Bottom dead center can be freely selected.    5. Stroke can be freely selected.
However, linear motors being used currently suffer from the shortcoming that they are incapable of generating large pressure. This means that conventionally it has been necessary to increase the number of linear motors in order to generate a large pressure.